Project: Inventory System

Time for a real project! We're going to build an inventory system: Tommy manages the station's supply depot, keeps track of provisions and needs to know what he's got and how much it's all worth

This project brings together everything you've learned: classes, constructors, ArrayLists, methods, and loops. Let's build it piece by piece

Step 1: The Item class

Every item in our inventory has a name, a quantity, and a price per unit. Let's model that

class Item {
    String name;
    int quantity;
    double price;

    Item(String name, int quantity, double price) {
        this.name = name;
        this.quantity = quantity;
        this.price = price;
    }
}

Simple and clean. An Item knows its name, how many we have, and what each one costs. The constructor takes all three values and sets them up

Step 2: The Inventory class

The Inventory class holds a list of items and provides methods to manage them. This is where ArrayList comes in: we don't know how many supplies will be stored, so a fixed-size array won't cut it

import java.util.ArrayList;

class Inventory {
    ArrayList<Item> items;

    Inventory() {
        items = new ArrayList<>();
    }

    void addItem(Item item) {
        items.add(item);
    }
}

The inventory starts empty. addItem adds an item to the list

Step 3: Removing items

For removeItem, we'll search by name and remove the first match. We need a regular for loop (not for-each) because we need the index to remove

public class Main {
    void removeItem(String name) {
        for (int i = 0; i < items.size(); i++) {
            if (items.get(i).name.equals(name)) {
                items.remove(i);
                return;
            }
        }
    }
}

We loop through items, check if the name matches, remove it, and return immediately (no point continuing after we found and removed it)

Step 4: Printing all items

Let's print each item in a nice format. This is where String.format comes in handy

public class Main {
    void printAll() {
        for (Item item : items) {
            System.out.println(item.name + " x" + item.quantity + " @ quot; + String.format("%.2f", item.price));
        }
    }
}

For an item named "Medical kit" with quantity 5 and price 25.00, this prints:

Medical kit x5 @ $25.00

Step 5: Calculating total value

The total value is the sum of quantity * price for every item. Classic loop-and-accumulate pattern

public class Main {
    double totalValue() {
        double total = 0;
        for (Item item : items) {
            total += item.quantity * item.price;
        }
        return total;
    }
}

Here's the full program put together

import java.util.ArrayList;

class Item {
    String name;
    int quantity;
    double price;

    Item(String name, int quantity, double price) {
        this.name = name;
        this.quantity = quantity;
        this.price = price;
    }
}

class Inventory {
    ArrayList<Item> items;

    Inventory() {
        items = new ArrayList<>();
    }

    void addItem(Item item) {
        items.add(item);
    }

    void removeItem(String name) {
        for (int i = 0; i < items.size(); i++) {
            if (items.get(i).name.equals(name)) {
                items.remove(i);
                return;
            }
        }
    }

    void printAll() {
        for (Item item : items) {
            System.out.println(item.name + " x" + item.quantity + " @ quot; + String.format("%.2f", item.price));
        }
    }

    double totalValue() {
        double total = 0;
        for (Item item : items) {
            total += item.quantity * item.price;
        }
        return total;
    }
}

public class Main {
    public static void main(String[] args) {
        Inventory inv = new Inventory();
        inv.addItem(new Item("Medical kit", 5, 25.00));
        inv.addItem(new Item("Oxygen reserve", 2, 75.50));
        inv.addItem(new Item("Solar battery", 1, 100.00));

        inv.printAll();
        System.out.println("Total: quot; + String.format("%.2f", inv.totalValue()));
    }
}

Output

Medical kit x5 @ $25.00
Oxygen reserve x2 @ $75.50
Solar battery x1 @ $100.00
Total: $376.00

Notice how the classes work together: Item is a simple data container, and Inventory manages a collection of items. This is composition: the Inventory has a list of Items. It's one of the most common patterns in real-world Java

Also notice: Item and Inventory are NOT public, only Main is public. That's because Java only allows one public class per file, and since the file is compiled as Main.java, the public class must be Main

Mission: Cargo Bay Manifest

The station's cargo bay needs a digital manifest system. Every supply item has a name, quantity, and unit price. Build the inventory tracker so the quartermaster can list everything and calculate the total value at a glance.

Create an Item class with fields: name (String), quantity (int), price (double) and a constructor
Create an Inventory class with an ArrayList<Item>, and methods: addItem(Item item), removeItem(String name), printAll(), and double totalValue()
printAll() prints each item as "name xQuantity @ $price" (use String.format("%.2f", price))
totalValue() returns the sum of quantity * price for all items
In main, store each supply's data in variables: name1/quantity1/price1, name2/quantity2/price2, name3/quantity3/price3 (start with Air filter/4/35.00, Food pack/10/12.00, Repair kit/3/85.00). Create an Inventory and add the three items built from those variables
Call printAll(), then print "Total: quot; followed by the formatted total

Output

Air filter x4 @ $35.00
Food pack x10 @ $12.00
Repair kit x3 @ $85.00
Total: $515.00